This invention relates to titanium alloy/fiber composite materials. In particular, this invention relates to a method for improving the fracture resistance of such composite materials.
Composites are recognized as a material class capable of operating under conditions requiring very high specific stiffness and strength. Synthetic matrix composites are generally limited to maximum operating temperatures of about 200.degree. C. Metal matrix composites are capable of higher operating temperatures. Aluminum- and titanium-based composites comprise the majority of metal matrix composites employed, particularly in aerospace applications. Aluminum-based composites are currently limited in application to about 800.degree. F., due to their degraded matrix strength at higher temperatures. Titanium-based composites are currently considered for many advanced aerospace applications in the temperature range of 800.degree.-1800.degree. F. due to improved matrix creep and environmental resistance.
Continuously reinforced conventional titanium matrices, e.g., Ti-6Al-4V and Ti-15V-3Al-3Cr-3Sn, have been the subject of numerous investigations. Metal matrix composites of these alloys have found limited applications in the temperature range of 800.degree.-1200.degree. F. Significant applications are under consideration for composites utilizing the ordered intermetallic matrices based on the Ti.sub.3 Al compound. This class of materials has greatly improved oxidation resistance as well as high temperature strength retention and is being considered for applications up to 1800.degree. F. In both classes of titanium composites, the fatigue properties in the direction of the reinforcement are reasonably good and represent improvements over the unreinforced materials. However, off-axis fracture properties are significantly reduced when compared to the monolithic (non-reinforced) alloys due to the poor load transfer at the interface, thereby limiting their application where isotropic properties are required. The composite fatigue properties have been shown to be controlled by matrix failure relatively early in life. It is assumed that these complex systems contain small defects in their as-fabricated condition. Such defects include reaction zone microcracks, reaction zone and matrix voids, matrix disbonds and cracked fibers. The fatigue life of the composite is then dictated by the time/load necessary to cause these flaws to propagate to a critical size wherein the composite fails. If the time/load required to reach this critical size is increased, the service life of the composite is similarly increased, particularly in applications requiring off-axis orientation loading.
Accordingly, it is an object of this invention to provide a method to increase the fracture resistance of titanium alloy matrix composites.
Other objects and advantages of the invention will be apparent to those skilled in the art.